The more I have read across these boards regarding this topic the more I am convinced that Mance has had a hand (direct or indirect) in bringing the north together (specifically regarding the Flints and Norreys, those clans) since before we knew of him in the books. I predict this 'Northern Conspiracy' goes back further then we know albeit it likely started for a different purpose and has changed as time, who is in charge, and events went on.
I am really looking forward to reading how this part of the story will conclude.
Might he still of been poisoned, only not directly by Oberyn and not at the time of that dinner?
I honestly don't think he had been poisoned but to only go after 1 instance when there was ample time with ample people who wanted Tywin dead since then seems a bit short sighted.
Personally I think his corpse just rotted fast as the conditions were right and/or he wasn't prepared after death properly (removing of the insides and blood being replaced, etc), maybe on purpose as a "fuck you Tywin" kind of thing.